Transistors such as metal oxide semiconductor field effect transistors (MOSFETs) or simply field effect transistors (FETs) or MOS transistors are the core building blocks of the vast majority of semiconductor integrated circuits (ICs). A FET includes source and drain regions between which a current can flow through a channel under the influence of a bias applied to a gate electrode that overlies the channel. The ICs are usually formed using both P-channel FETs (PMOS transistors or PFETs) and N-channel FETs (NMOS transistors or NFETs) and the IC is then referred to as a complementary MOS or CMOS circuit. Some semiconductor ICs, such as high performance microprocessors, can include millions of FETs. For such ICs, decreasing transistor size and thus increasing transistor density has traditionally been a high priority in the semiconductor manufacturing industry. Transistor performance, however, must be maintained even as the device size decreases.
In some integrated circuit designs there has been a desire to eliminate the use of polysilicon gate electrodes to improve device performance with decreased feature sizes. Replacing polysilicon gate structures with metal gate structures is one solution. Often, metal gate structures are formed in trenches in FET device regions and utilize aluminum or tungsten as a metal fill portion in the trenches in conjunction with a work function metal layer(s). The aluminum or tungsten metal fill portion is used as a conductive metal fill to offset the relatively higher resistance of the work function metal layer(s) to lower the overall resistance of the metal gate structure. However, as device sizes get smaller, metal gate structures with lower resistance than conventional metal gate structures formed with aluminum or tungsten metal fills are needed. Copper exhibits lower resistance than aluminum and tungsten. Unfortunately, copper typically forms conductive metal fill with voids when deposited in relatively small trenches. The presence of voids within the copper fill can increase the resistance of the metal gate structure and adversely affect the electrical characteristics of the resulting device.
Accordingly, it is desirable to provide methods for fabricating integrated circuits having low resistance metal gate structures. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.